Heating appliance

ABSTRACT

A domestic combined heat and power system comprising a Stirling engine ( 1 ) and water heater in the form of a supplementary burner ( 17 ). The exhaust gas from the Stirling engine is used to preheat combustible gas entering the Stirling engine and subsequently used to heat the water. The water heater ( 15 ) has a helical water duct ( 41 ) towards the periphery of a housing ( 39 ). Separate parts of this duct are heated, in series, by the exhaust gas from the Stirling engine and the supplementary burner ( 17 ) firing radially outwardly through the helical duct.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to Great Britain Application No.0130380.9, filed Dec. 19, 2001, which application is incorporated hereinfully by this reference.

The present invention relates to a heating appliance. In particular, theinvention relates to a domestic combined heat and power (DCHP) unit.

Such DCHP units have been proposed which incorporate a Stirling engineconnected to an alternator to generate electricity. The heat produced bythe engine which would otherwise be wasted is used to heat water feedingthe domestic water and central heating systems, and therefore becomes avaluable product of the DCHP system. It has been shown that the mostfavourable economics are achieved for the unit when the Stirlingengine/alternator is sized to generate approximately 1 kW ofelectricity. At this level, however, only approximately 5 kW of heatwill be provided, which is substantially below a typical domestic heatload of in excess of 20 kW.

In order to supply the remaining heat, so that the DCHP unit can competeeffectively with the current specification of a domestic boiler, asupplementary gas burner is required. The present invention aims toprovide a thermally efficient operation of the Stirling engine andsupplementary burner.

According to the present invention there is provided a domestic combinedheat and power assembly comprising a Stirling engine and a water heaterwherein the Stirling engine is arranged to be heated by a first burnerfed with combustible gas, the assembly further comprising an exhaust gasduct passing from the Stirling engine in contact with a combustible gasinlet into the first burner to preheat the combustible gas entering thefirst burner and subsequently heat water which is additionally heated bythe water heater, wherein the water heater is provided is provided by asecond burner, and wherein the system is arranged so that the exhaustgas and gas from the second burner form a combined stream once they havegiven up heat to the water and the combined stream is arranged to heatthe water upstream of the exhaust gas.

Also, the water will receive relatively low grade heat from the combinedstream, as the components of the combined stream have already give upsome heat to the water. The water will subsequently receive relativelyhigher grade heat directly from the exhaust gas and from thesupplementary burner. Such an arrangement is thermally efficient.

Preferably the system is arranged so that the exhaust gas and gas fromthe supplementary burner form a combined stream once they have given upheat to the water and the combined stream is arranged to heat the waterupstream of the exhaust gas. With such an arrangement, the water willreceive relatively low grade heat from the combined stream, as thecomponents of the combined stream have already given up some heat to thewater. The water will then subsequently receive relatively higher gradeheat directly from the exhaust gas and from the supplementary burner.Such an arrangement is thermally efficient.

Preferably the system further comprises a Stirling engine coolerarranged to heat the water upstream of the exhaust gas heating. This notonly serves to promote the heating of the water, but also helps tomaintain the required temperature differential across the Stirlingengine.

The burner gases heating the head of the Stirling engine are typicallyat around 1400° C. As the gases flow around the head, heat istransferred into the heater head, and the temperature of the gases fallsto around 800° C. The gases transfer heat to the incoming fuel/airmixture. If there is no further heat recovery, the exhaust gases flowinginto the Stirling engine flue then typically have a temperature ofaround 600° C.

The present invention also extends to a novel design of heatingarrangement to transfer heat into the water. The requirements for thisare that it is compact and is able to transfer heat from at least twostreams into the water at maximum efficiency.

According to a further aspect of the present invention there is provideda heating arrangement for heating a fluid, the arrangement comprising ahousing; a duct within the housing towards its outer periphery andhelically wound about a central axis of the housing to convey the fluidfrom the first end of the housing to a second end of the housing; asupplementary burner in the housing adjacent to the second end andsurrounded by the duct to transfer heat to the fluid in a first portionof the duct; an outlet for hot gas from an external heat source, theinlet being configured to direct the hot gas radially outwardly onto asecond portion of the duct closer to the first end of the housing thanis the first portion of the duct.

Such an arrangement is particularly compact as the supplementary burnerand inlet for gas from an external source are within the duct throughwhich the fluid is conveyed. Further, the supplementary burner and inletare arranged to transfer heat to different parts of the helical duct.Thus, the arrangement can be set up to be particularly thermallyefficient.

Preferably the inlet is arranged to substantially prevent mixing of thegas from the supplementary burner and the hot gas from the external heatsource at least until both have passed through the helically wound duct.Thus the streams are substantially prevented from mixing prior toreaching the duct ensuring a more efficient heat transfer.

Once the exhaust gas and hot gas from the external source have passedthrough the helically wound duct, they may then simply be dischargedeither separately or as a combined stream. However, preferably, the twostreams are formed into a combined stream once they have initially givenup some heat to the duct, and the combined stream is passed around athird portion of the duct closer to the first end than is the secondportion of the duct. Thus, the cool fluid in the helically wound ductwhich enters the first end of the housing initially encounters thisrelatively cool combined stream and is preheated by this stream beforebeing heated by the gas from the external heat source and the gas fromthe supplementary burner.

Although some mixing of the gas from the supplementary burner and thehot gas from the external heat source is tolerable as a small amount ofmixing will have relatively little effect on the overall heating of thefluid, it is preferable to minimise such mixing as much as possible.Therefore, preferably, a seal is provided to prevent mixing of gas fromthe supplementary burner and the hot gas from the external sourceradially inwardly of the duct.

In order to maximise the heat transfer to the duct, the duct ispreferably arranged such that each coil of the duct is flattened in thedirection of the central axis so that, in cross section, its radialdimension exceeds its axial dimension. Also, preferably, adjacent coilsof the duct are closely adjacent to or abut one another. This ensuresthat the hot gas has to pass through a relatively long and narrowpassageway between adjacent coils of the duct which makes for efficientheat transfer. This duct configuration is shown in WO 94/16272.

An example of the invention will now be described with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic diagram showing the layout of a first Stirlingengine system;

FIG. 2 is a schematic diagram showing the layout of a second Stirlingengine system;

FIG. 3 is a cross-section through a heating arrangement; and

FIG. 4 is an end view of the arrangement of FIG. 3.

The arrangement shown in FIG. 1 comprises a Stirling engine 1 to whichheat is supplied by a burner 14. The heat is transferred to the head bya system of fins 3 as disclosed in our earlier co-pending applicationno. 0020012 and exits through a flue.

The arrangement in FIG. 1 has a water heater 15 which is arranged toheat a water stream 16 by virtue of a supplementary burner 17. Exhaustgases are expelled through flue 18.

A common gas supply line 19 is provided for both the first 14 andsupplementary 17 burners. This gas flow is controlled by a singlemulti-functional valve 20. The burner controls controlling ignition andflame detection of the burner may be incorporated into amulti-functional controller. Gas in the multi-functional valve 20 entersa mixing chamber in the form of a venturi manifold 21 and is mixed withan air stream 22 driven by a fan.

The gas and air mixture is now supplied to a splitter valve 23 in whichone inlet 24 leads to a first burner outlet 25 and a second burneroutlet 26. The relative amount of flow to each outlet is controlled by aflap valve 27 which is rotatable about an axis 28. The position of theflap valve 27 will be set by the appliance controller, and is driven tothat position, for example, by a servo motor. Alternatively the air flowcould be divided after a single intake fan 60, by such a splitter valve23, and then each air stream could incorporate a multi-functionalvalve/controller to accurately meter the gas mixture to each burner.This arrangement is shown in FIG. 2.

The fuel/air mixture fed to the Stirling engine 1 flows around a cowling29 enclosing the burner arrangement and is hence heated by the hot gasstream leaving the burner. The exhaust gas which has given up some ofits heat to the incoming mixture leaves the cowling through manifold 30and enters the water heater 15 where it comes into contact with thewater stream 16 (which has already been heated to some extent in theStirling engine cooler 31) so as to further preheat the water stream 16upstream of the supplementary burner 17. The supplementary burner 17 isfired to provide supplementary heating to the water stream 16 ifrequired. The exhaust gas from the first burner 14 leaves the waterheater 15 with the exhaust gas from the supplementary burner 17 throughflue 18.

The heating arrangement is shown in more detail in FIGS. 3 and 4 andcomprises the supplementary burner 17 and water heater 15. The waterheater 15 has a generally cylindrical housing 39 with a main axis 40.The supplementary burner 17 is positioned on the axis 40 so as to fireradially outwardly.

Water is fed through the water heater 15 from the right hand side ofFIG. 3 to the left hand side around a water duct 41 Which is a singlehelical tube (which may be made up of a number of connected segments)wound around the axis 40 towards the outer periphery of the cylindricalhousing 39. Each individual coil of the water duct 41 is flattened inthe axial direction. Adjacent coils are positioned closely adjacent toone another, but are not sealed together such that hot gas can passbetween adjacent coils. Extending towards the middle of the heater 15 isan inlet 42 for the exhaust gas from the Stirling engine. The inletleads to a circular chamber 43 defined between two circular plates44,45, the latter of which is an opening for the inlet 42. Annularbrackets 46 bridge the gap between the edge of the plates 44,45 and thewater duct 41 to ensure that all the exhaust gas from chamber 43 passesthrough the coils of the water duct 41. Towards the second end from thechamber 43 is a supplementary burner chamber 47 into which thesupplementary burner 17 fires. The supplementary burner heats theadjacent coils towards the second end.

Towards the first end from the chamber 43 is a discharge chamber 48 withan outlet 49. There is also an outlet 50 for condensate produced due tothe high efficiency nature of this final stage of heat recovery.

The operation of the water heater 15 is as follows. Water flows throughthe water duct 41 from the first end to the second end around a helicalpath. The supplementary burner 17 fires into chamber 47 and exhaust gasenters at inlet 42 and passes into chamber 43. These two hot streamspass radially through gaps between adjacent coils in the water duct 41into an outer annular chamber 51 and form a combined stream. Thiscombined stream then flows back through the water duct 41 adjacent tothe first end as indicated by arrow 52. The incoming water is thereforeinitially heated by this combined stream. As it flows further towardsthe second end it encounters the Stirling engine exhaust gas from thechamber 43 and is heated further. Finally, it encounters the hot gasfrom the supplementary burner 17 to provide a third stage heating.

By providing this three stage heating arrangement, efficient heating ofthe water is provided. Further, as can be appreciated from FIG. 2, thearrangement in which this three stage heating is provided isparticularly compact resulting in cost and space saving.

1. A domestic combined heat and power system comprising a Stirlingengine and a water heater, wherein the Stirling engine is arranged to beheated by a first burner fed with a combustible gas, the assemblyfurther comprising an exhaust gas duct passing from the Stirling enginein contact with a combustible gas inlet into the first burner to preheatcombustible gas entering the first burner, and subsequently heat waterwhich is subsequently heated by the water heater, wherein the waterheater is provided by a second burner and wherein the system is arrangedso that the exhaust gas and gas from the second burner form a combinedstream once they have given up heat to the water and the combined streamis arranged to heat the water upstream of the exhaust.
 2. A systemaccording to claim 1, further comprising a Stirling engine coolerarranged to heat the water upstream of the exhaust gas heating.